1. Field Of The Invention
The present invention relates to a method and apparatus for determining track eccentricity of an information-bearing disk.
2. Description Relative To The Background Art
In the following description of relevant background art, reference is made to FIG. 1 of the accompanying drawings which illustrates eccentricity of a record track of a disk system.
With reference to the recording art, tracking is the process of keeping a transducer head on the path of a record track on a storage device. The purpose of tracking control is to adjust the position of the head relative to the record track or vice versa, so that the head is aligned with the centerline of the track for maximizing the signal-to-noise ratio of a record/playback channel.
Of the various causes of tracking error, track eccentricity is often the most severe in a disk system. The problem arises when the center of a disk is not aligned precisely with the rotational axis of a disk drive mechanism. As a result, a track being traced oscillates radially as the disk rotates about its central axis of rotation.
FIG. 1 illustrates the problem. A disk drive spindle 10 has a central axis of rotation 11 at point A; a stationary transducer head 12, located at point B a known distance, R, from A, serves to trace an imaginary circular track 14 of radius R passing through B and centered at A.
Track eccentricity arises when a disk 16 has its center (point C) displaced from the rotational axis 11 at A. Point C is shown generally at a distance epsilon, .epsilon., from A and at a phase angle phi, .phi., from a radial line 17 related to a predetermined reference position of the rotational axis 11.
The disk 16 has a circular record track 19 (center at C) whose radius is the distance R that the transducer head 12 is from A. If point C were to coincide with A, the record track 19 would be and would remain aligned with the imaginary track 14 when the disk 16 rotates. If that were the case, there would be, of course, no track eccentricity.
With miscentering, however, the center of the disk 16 (point C) follows a circular path 20 of radius .epsilon. and center at A as the disk 16 rotates. As C follows path 20, the track 19 moves radially relative to point B of the transducer head 12. A distance, .DELTA., between B and the point at which track 19 intersects a diametrical line 18 passing through A and B, represents a tracking error caused solely by eccentricity. The distance .DELTA. varies as a function of the location of point C on the path 20 and is at a maximum .epsilon. outside point B when C is aligned with line 18 between A and B and is at maximum .epsilon. inside B when C coincides with line 18 on the diametrically opposite side of A. Thus, the point of intersection of the track 19 with the line 18 wanders back and forth within a total range of 2.epsilon.. In this case a sinusoidal signal of amplitude .epsilon. and having a fundamental frequency component equal to the rotational speed of the disk and proper phase provides a very good approximation to track eccentricity.
It should be understood that the center C of the disk 16 is shown to be a relatively significant distance from the rotational axis 11 of the drive spindle 10. In actuality, the distance .epsilon. is typically no greater than 25 microns--a small percentage of the track radius R.
There are known in the prior art various techniques for achieving tracking control dynamically. A commonly employed method is to preformat a disk with so-called embedded servo information. By this procedure, predetermined signals are recorded along each record track on a disk at precise locations. As these signals are detected, either in a record or a playback mode, a servo system, preferably closed-loop, serves to drive the transducer head radially in an attempt to maintain alignment with the particular record track being traced. Embedded servo information has been found to work well with disks having information-bearing tracks of moderate density such as a rigid magnetic disk. In fact, for a disk system having a relatively low track density, such as is found with some flexible or floppy disk systems, tracking control may be achieved without embedded servo information. A problem arises, however, with a disk system having a relatively high track density, such as a magneto-optical or an optical system, in which track density may be on the order of approximately 6,000 tracks per centimeter. With a system of this type wherein track pitch is less than two microns (&lt;2.mu.), a discrete tracking signal provided by embedded servo information is not sufficient in itself to maintain tracking control. This is because eccentricity can cause the track to wander radially inwardly and outwardly relative to a transducer head over an annular band containing several concentric tracks. A tracking error of this magnitude can cause complete loss of a desired track during the time interval between adjacent servo signals. With a disk system in which track density is particularly high, eccentricity must be continuously compensated for during operation.
U.S. Pat. No. 4,628,379 discloses apparatus for continuously compensating for eccentricity of a disk storage system. To that end, servo information, stored at discrete points angularly spaced equally relative to the center of the disk, provides information relating the position of a transducer head to disk center. To provide for a continuous compensating factor, a microprocessor uses a discrete Fourier transform technique to fit detected servo information to a sinusoidal waveform defining eccentricity.
A discrete Fourier transform analysis, however, suffers from a disadvantage in that the position information for computing eccentricity is required to be provided at equally spaced points in time. To that end, U.S. Pat. No. 4,628,379 provides servo information aligned in the radial direction. The arrangement of servo information in this pattern is commonly associated with a constant angular velocity (CAV) system. In such a system a disk has the same number of bit cells in each track revolution. Because the length of each track revolution depends on its radius, bit cell density decreases with increasing radius and only the innermost track may have a maximum allowable bit density. Thus, an eccentricity compensating scheme based on a Fourier analysis technique suffers from disadvantages in that it requires servo information arranged in a particular time-ordered pattern which itself is very inefficient in terms of data storage capacity.